54 HEREDITY 



What is the explanation of this ratio ? The main 

 points were given by Mendel himself, though his theory 

 has been somewhat modified by later writers. It is 

 supposed that the tall pea, for instance, possesses some 

 factor by virtue of which it is tall, while this factor is 

 absent in the dwarf form. All the reproductive cells 

 (ovules and pollen-grains) of the tall pea will possess 

 this factor, while all those of the dwarf will lack it. 

 The hybrid between the two forms is produced by the 

 union of these two different types of reproductive cells. 

 The degree of dominance will depend on the relative 

 potency of the factor for taUness when present in the 

 hybrid condition that is, when it has been introduced 

 through only one of the reproductive cells, instead of 

 through both. 



The essential point is now that the reproductive cells 

 of the hybrid plant do not all receive an " average 

 sample " of the hybrid germ plasm, but that half of 

 them come to possess the factor for tallness, while the 

 other half lack it. The " factor " is not divisible. 

 There is no question of all the reproductive cells bearing 

 " a certain amount of tallness.'" The factor must either 

 be present or absent, and as a matter of fact it is present 

 in half and absent in half of the reproductive bodies. 

 With regard to any particular character the individual 

 produces germ cells of the same kind as those from which 

 it itself arose. Our hybrid tall plant arose from the 

 union of a " tall " and a " non-tall " reproductive cells, 

 and it again gives rise to these two types in equal 

 numbers. 



But how is the Mendelian ratio produced ? We have 

 two kinds of female reproductive cells, and two kinds 

 of male. Their union takes place according to what 

 we call " pure chance." If T represent a reproductive 

 cell containing the factor for tallness, and t be taken 

 to represent one in which the factor is absent, then we 

 have T and t pollen-grains and T and t egg-cells in 

 equal numbers. Supposing a certain pollen-grain T, 

 it is evidently an even chance whether it unites with an 

 egg-cell T or an egg-cell t. We therefore get the two 



